A. Field of the Invention
This invention relates to the separation of hydrogen sulfide from a mixture of gases which could include methane, and, more particularly, to multi-component membranes for separating hydrogen sulfide from gaseous mixtures, and to processes and apparatus for separating hydrogen sulfide from gaseous mixtures by permeation utilizing such multi-component membranes.
B. Background Art
The use of semi-permeable membranes for reverse osmosis or ultra filtration processes is well known. For example, in a reverse osmosis process, high pressure saline water is placed in contact with a semi-permeable membrane permeable to water but relatively impermeable to salt in order to separate concentrated brine and relatively pure water, the water then being available for personal use such as drinking, cooking, and cleaning.
It has now been discovered that certain membranes may also be employed for the separation of various specific gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of a gas mixture across a surface of the membrane at an elevated pressure relative to an effluent stream emerging from the other surface thereof. Any component of the mixture which is more permeable than the other gases thereof will pass through the membrane at a more rapid rate than the less permeable components. Therefore, the permeate stream which emerges from the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable components of the feed gas mixture. Thus, selective separation can provide preferential depletion or concentration of one or more desired gases in a mixture.
This invention is particularly concerned with a membrane which is more permeable to hydrogen sulfide than to other gases with which it might normally be mixed. Through the use of the membrane of the present invention, the permeate stream passing through the membrane exhibits an enriched concentration of hydrogen sulfide in relation to the feed stream of gases, while the residue of the feed stream exhibits a decreased concentration of hydrogen sulfide gas.
Many possible uses of such a membrane are conceivable. Reduction of air pollution makes it essential to minimize the release of sulphur dioxide into the atmosphere. By removing hydrogen sulfide from coal gas utilized in combustion processes, oxidation of the hydrogen sulfide into sulphur dioxide is avoided. Moreover, in many commercial enterprises, the waste gas from hydrogen sulfide removal is converted into sulphur in a process the cost of which is inversely dependent upon the concentration of hydrogen sulfide supplied. As an overall effect, therefore, removing hydrogen sulfide prior to combustion of coal gas not only will reduce air pollution resulting from that combustion, but can, through concentration of the hydrogen sulfide stream, result in lowering the cost of processing waste gases.
Furthermore, natural gas contains various percentages of hydrogen sulfide. To be commercially acceptable, however, the hydrogen sulfide content of natural gas must be reduced to concentrations of no more than one quarter to one half grain per one hundred standard cubic feet, so as to minimize the risk of hydrogen sulfide corrosion of valves and fittings in natural gas distribution systems.
Hydrogen sulfide may be removed from hydrocarbon gas streams such as natural gas by many methods. These methods may be broadly classified as chemical reaction, physical absorption, and adsorption. Chemical reaction processes rely on reversible chemical reactions and use an absorbant which reacts with hydrogen sulfide in a contactor. The absorbant can be regenerated by use of a high temperature stripper. The reversal of some chemical reaction processes is so difficult that cost prohibits regeneration, and hydrogen sulfide is removed in a precipitation process which consumes the absorbant, usually a heavy metal chloride or nitrate. The physical absorption processes utilize the affinity of certain chemicals for hydrogen sulfide and basically employ a contactor to remove acid gas from the feedstream. Also, a stripper is used to separate the acid gas from the absorbent. The adsorption processes are based on the unique adsorbant qualities of certain minerals such as zeolites. Generally, these adsorption processes are of a batch-type employing a molecular sieve. In operation the acid gas components of the feed gas stream are adsorbed on the surface of the mineral used and are subsequently removed therefrom during a high temperature regeneration cycle.
All of the above-mentioned processes are not particularly attractive when evaluated using commercial parameters such as cost, energy consumption, plant area requirements, operation manpower requirements, and maintenance costs. These processes become more uneconomical for treating sour natural gas as the cost of the processes, evaluated with the above paramaters continue to increase. For example, on the north slope of Alaska and on offshore platforms, the area available for process systems is extremely expensive and, hence, it follows that systems used at these locations must have small area requirements.
Furthermore, it is well-known by those in the art that these processes are energy-intensive. Molecular sieves, for example, must be heated to and held at approximately 600.degree. Fahrenheit during regeneration in order to remove all of the adsorbed materials from the mineral surfaces. High energy input is required to achieve such temperatures. An additional disadvantage of the above-mentioned methods is that they quite frequently require interruption of the separation process to permit regeneration and/or replacement of the chemicals involved; therefore, truly continuous flow-through separation processes are not available.
However, other processes have been used to separate one or more gaseous components from a gaseous mixture. In particular, membranes have been used for many years in gas permeation separation methods. Gas permeation may be defined as a physical phenomenon in which certain components selectively pass through a substance such as a membrane. Basically, a gas permeation process involves introducing a gas into one side of a chamber which is separated into two compartments by a permeable membrane. The feed gas stream flows along the surface of the membrane and its more permeable components pass through the membrane barrier at a higher rate than those of lower permeability. After contacting the membrane, the depleted feed gas residue stream is removed through a suitable outlet on the feed compartment side of the vessel. The other side of the membrane, the permeate side, is provided with a suitable outlet through which the permeate gaseous components can be removed.
The purpose of a membrane in a gas permeation process is to act as a selective barrier, that is, to permit passage of some but not all components of the gaseous feed stream. Generally, in gaseous membrane separation processes, the separation is due to molecular interaction between gaseous components of the feed stream and the membrane. Because different gaseous components react differently with the membrane, the transmission rates are different for each gas. Hence, separation of different components can be effected by single or repeated diffusions through a given selective membrane.
To date the selection of suitable components for use in a gas separation membrane is largely a matter of intuition.
Accordingly, one object of the present invention is the production of a membrane suitable for use in a continuous flowthrough process for extracting hydrogen sulfide from a mixture of gases.
Another object of the present invention is a selective gas permeation membrane which favors hydrogen sulfide diffusion and is possessed of durable construction and capable of being manufactured with ease from readily available components.
Additional objects and advantages of the invention will be set forth in a description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.